Originally Posted by mstateglfr

You are brushing on the paint?...how much of the frame is in need of paint? The pic you provided is just a few scratches which could be touched up with a paint pen.

As mentioned, get rid of any rust before painting. Evaporust is available at car parts stores and doesnt harm paint. soak paper towels in the solution, wrap the tubes tight, then cover with saran wrap to keep the towel from drying out. Wait a few hours and the surface rust will be gone. Repeat wherever necessary. Its extremely minimal work and you can use the rest of the Evaporust to get rid of the surface rust on your components(if there is any).

Be careful of white. There are seemingly infinite shades of white. 30 year old white is also faded and often times a new white will obviously clash. You have to adjust and buy an offwhite to match what was originally white.

A paint pen or a can of testor's paint in the correct shade with a toothpick or something really small to drop the paint in the scratch will be the least invasive. Paint, being a liquid, will fill the scratch if dropped in versus brushed on, and you then dont have brush lines.

As for your specific questions, if its touch ups just use the paint on the frame. If the frame is clean and dry itll stick fine. Again, less invasive than primer and paint.
...if I completely misunderstood your post, sorry.